The present invention is directed to an apparatus for processing a dead pixel, more specifically to an apparatus for detecting and compensating a dead pixel that can maintain excellent image quality by reducing image distortion and image deterioration.
An image sensor has a two-dimensional arrangement of pixels, each of which transforms light to an electrical signal according to the luminosity. By measuring the electrical signal, the intensity of light entered into each pixel can be inferred, and using this, images made up of pixels can be constructed.
As such, an image sensor comprises pixel arrays, which consists of several hundred thousand to several million pixels, a device to convert analog data sensed from a pixel to digital data, and several hundred to several thousand storage devices. Due to the large number of devices, an image sensor is always exposed to a possibility of making an error in the process, and such an error can cause dead pixels, which become an important factor determining the level and price of the image sensor. The pixel value resulted from a dead pixel is characterized by being exceedingly larger or smaller than the pixel values of neighboring pixels.
FIG. 1 shows a method for detecting and compensating dead pixels in accordance with the prior art. As shown in FIG. 1, in case an image sensor outputs an electrical signal (i.e. a sensor output), corresponding to an optical image inputted through a lens, like the reference number 110, the pixel value corresponding to each pixel is stored in a register temporarily, processed for dead pixel detection and compensation, converted to a corresponding electrical signal, and then sent to an image processing device.
Once the electrical signals 110 corresponding to an optical image inputted through a lens are converted to digital signals by the image sensor and outputted, the register 115 receives and stores the outputted digital signals in sequence. The register stores the pixel value corresponding to the order of R-G-R-G or B-G-B-G. Each pixel value temporarily stored in the register 115 is outputted to an imaging processing device in the same order of input, through the first-in-first-out method.
In this process, the pixel value of what is recognized as a dead pixel is detected and compensated in the following process: For example, in order to determine if the pixel value stored in G2 corresponds to a dead pixel, it is determined if the difference between the pixel value (VG2) of a pixel stored in G2 and the pixel value (VG1) of a pixel of the same kind stored in the following, neighboring location satisfies the condition of |VG2−VG1|>the reference value, and if the difference between the pixel value (VG2) of a pixel stored in G2 and the pixel value (VG3) of a pixel of the same kind stored in the preceding, neighboring location (G3) satisfies the condition of |VG2−VG3|>the reference value.
If the pixel value stored in G2 does not satisfy any one of the above equations, the pertinent pixel value (VG2) is perceived to be normal, and thus no compensation is made to the pixel value of the pertinent location (step 125). However, if the pixel value stored in G2 satisfies all of the above equations, the pertinent pixel value (VG2) is perceived to be a dead pixel, and is replaced using the data of preceding and following pixels. And through the process described above, the pixel value stored in G2 is compensated and is converted to an electrical signal 130 corresponding to the compensated pixel value to be sent to an image processing device.
U.S. Pat. No. 6,965,395, issued to Neter, discloses one of the conventional method. The prior art reference discloses a color imaging system which employs an on-the-fly bad pixel detection and correction process using a signal processing procedure performed during readout of the imaging device output signals, while those signals are still in analog form. This on-the-fly bad pixel detection and correction process is capable of random access readout and includes programmable gain amplifiers, an A/D converter, one or more registers for temporary storage, and a signal processing block. In this on-the-fly mode of operation, the signal processing unit performs the detection step via horizontal, vertical and diagonal conditional median filters. If the pixel whose value is being examined, which may be the middle or center pixel within a horizontal, vertical or diagonal set of pixels, the is not within the condition limits, its value is replaced in the correction step. The replacement value may be determined by the directional median that possesses the minimum variation. The value chosen can be, by way of example, a median, weighted mean, or average value of two or more pixels in the neighborhood in which the bad pixel is situated.
However, the conventional methods described above have the problem of using too much hardware resources. Besides, crowded images or high-contrast images can be erroneously perceived as dead pixels, resulting in unnecessary compensation. The recovered image, therefore, has much more distortion than the actual image.
Further objectives and advantages of the present invention will become apparent from a careful reading of a detailed description provided hereinbelow, with appropriate reference to the accompanying drawings.